For brevity's sake, reference will be made in the following only to the processing of polyester fibres, which are the most widely produced fibres—from a quantitative point of view—on the market and hence those attracting the highest economic interest. However, the present invention can be applied, with equally satisfactory results, to any chemical fibre belonging to the above-listed general categories, and it is hence in no way limited to the use with polyester fibres, specifically cited here only to illustrate and exemplify the invention.
The crimping operation lies halfway through the manufacture of polyester (PET) fibres, whose different stages will now be briefly summarised to better set the field of the technique in which the present invention is situated.
The raw material, consisting of polyester in the shape of chips or melted polymer, is produced in poly-condensation plants, generally from terephthalic acid and ethyleneglycol. In the case of chips, the dried, solid polymer is sent to extruders where the material is melted and fed to spinning pumps.
The pumps, which are volumetric pumps, feed high-pressure, melted material to spinnerets, each provided with several thousands holes with diameters under one millimeter. From the spinnerets, continuous filaments come out in a melted condition, said filaments being solidified and cooled by flow-controlled, humidity-controlled and temperature-controlled air. A spinning plant usually consists of a few up to several dozens spinnerets. Each spinneret hence produces a band of continuous filaments, which are collected by rolls and gathered together to form a single subtow, which is subsequently laid down by reels, line reels into suitable collection containers.
Due to processing requirements, during spinning the filaments are manufactured at a speed of 1.000-2.000 m/min, while the second stage of the process—which comprises drawing and stabilisation of the filaments, crimping and flock cutting—has a maximum speed of 300-400 m/min. The two stages of the process are hence separate and for this reason the subtows manufactured in the spinning stage are temporarily laid down by respective line reels into suitable collection containers, wherefrom the subtows are then taken by creels for the next processing.
Unlike the spinning stage, which occurs continuously except for the programmed maintenance/cleaning halts or those due to breakdowns, the second manufacturing stage is discontinuous and in any cycle—which may last from a few hours up to a whole day—continuous operation accounts for about 80% of the cycle and then a dwell time accounts for the remaining part of the cycle to carry out manufacture changes.
In this second stage, a certain number of subtows of polyester fibre, taken from the collection containers by a creel, are joined together to form, typically, 3 tows of the desired count; these tows are subsequently washed in a swilling and oiling tank, to remove the finish applied during the spinning stage, and to oil the fibres.
Subsequently, the tows thus treated undergo a multiple-step drawing operation until they reach the desired weight by length unit (deniering). The drawing is accomplished by drawing assemblies and each assembly generally consists of 7 or more rolls, rotating at increasing peripheral speeds for each assembly.
The first drawing is generally accomplished in hot water between the first pair of drawing assemblies; in this stage typically about 80% of the final, achievable drawing is obtained. The second stage is generally performed in steam between the second and the third drawing assembly. The overall drawing thereby reaches 400÷600%. The drawing operation orientates the molecules according to the longitudinal direction and imparts to the fibres the necessary weaving properties.
After drawing, the tows are oiled again by immersion and then treated by a set of hot drums, generally between 8 and 24 in number. On these drums the tows are dried and heated up to about 190° C. to fix the drawing effect while under tension. At the exit from the hot drums, the tows are cooled by a set of cooling drums fed with controlled-temperature water. In order to ensure that the tows are constantly under tension, sometimes, after the cooling drums a last set of drawing drums is arranged.
The tow finish, i.e. the oiling or final finishing treatment, is typically performed by a spray-type unit, provided with a dosing pump for the administration of a precise amount of size.
The three tows thus treated are finally sent to a tow overlapping unit, consisting of multiple drums, which overlaps the previously-treated tows to form a single, extended tow, having a substantially rectangular section and a width suitable for feeding it to the crimping machine. Inside the crimping machine the deformation of the individual filaments occurs by mechanical collapse due to point load of the filaments, pushed at high speed into a confined chamber. Current crimping machines operate at a maximum speed of about 400 m/min and hence represent the “slow stage” of the PET fibre manufacturing process, even though the high number of filaments they treat simultaneously allows to obtain an overall balancing of the respective capabilities between the first and the second stage of the above-described manufacturing process.
Downstream of the crimping operation, the crimped tow enters a drying oven where free-fibre (untensioned) stabilisation occurs to allow retraction thereof. The oven consists of multiple heating units and of a cooling unit. Free-fibre stabilisation makes the crimps produced by the crimping machine permanent and improves elongation to rupture.
At the exit from the drying oven, a tensioning wheel collects the tow and keeps constant the tension of the material entering a cutter, where the tow is cut into fibres of preset length (for example, typically 38 mm in the case of spun polyester) and thereby processed into flock. The product is subsequently sent for packaging.
The quality of the end product of the above-described process is largely determined by the performance of the crimping machine, so that stability and repeatability of the crimping process end up significantly affecting the entire manufacturing process of polyester fibres. As a matter of fact, while the spinning, drawing and stabilising stages have by now reached a high degree of improvement and can therefore be correctly reproduced and controlled, the crimping stage is currently still in a fully empirical stage and, in particular, no adequate control system has been devised yet which allows a reasonable forecast of the machine behaviours, on the one hand to avoid the production of insufficient or low-quality (i.e. uneven) crimped material and, on the other hand, to avoid jamming or blockages of the crimping machine.